The prior art is depicted in FIGS. 1 through 10.
FIG. 1 depicts a prior art passive wireless SAW (surface acoustic wave) sensor, an antenna and reflectors disposed on a material of interest, e.g., metal, concrete, plastic or any material that propagates RF signals. An RF interrogating signal is received by the sensor and responsive thereto a SAW transducer creates a SAW wave. The SAW wave signal propagates through the material and is reflected from the reflectors back to the antenna. From the antenna the reflected signal is propagated to the remote interrogating antenna for processing.
Characteristics of the reflected signal (e.g., time delay, propagation losses, phase delay) indicate certain characteristics of the material of interest, including a material temperature, forces, stresses, strains, etc. exerted on the material.
FIG. 2 depicts a wireless passive SAW sensor network and its components comprising: an RF wireless SAW interrogator, a controlling computer (depicted as a laptop computer in FIG. 2), an interrogating antenna for transmitting the interrogating signal to the SAW sensor and receiving the response from the sensor, and a plurality of remote SAW sensors (four illustrated) disposed in or on a material of interest.
FIG. 3 illustrates RF SAW sensor responses from a four-sensor network system of FIG. 2. The sensor echoes are in this case separated in time, but may overlap if certain signal coding schemes are used. This system can function with any number of sensors and is thus not limited to four sensors.
FIG. 4 illustrates an RF return echo of a single sensor in a system comprising a plurality of sensors, such as the sensor system of FIG. 2. Again, the number of sensors is limited only by desired performance, application, complexity, and cost.
FIGS. 5(a) through 5(d) illustrate several exemplary dipole PCB (printed circuit board) antennas for construction on a printed circuit board substrate. The SAW device is typically installed in a chip carrier package with two leads exiting the package for soldering to each of the two illustrated antenna terminals 20 and 21.
FIG. 5(a) depicts a common linear dipole antenna having a trace width W that affects the sensor bandwidth and a length λ/2 that affects the antenna gain and resonant frequency.
FIG. 5(b) depicts a dual slope slanted dipole antenna.
FIG. 5(c) depicts a single slope slanted dipole antenna.
FIG. 5(d) depicts a folded dipole antenna.
FIG. 6 depicts a SAW device 22 comprising a transducer 24 mounted on a substrate 25 and attached through two bond wires 28 and 29 to two pins 30 and 32 of a chip carrier 33 (header), a plurality of reflectors 35, and a lid 36 for hermetically sealing to a chip carrier package 38.
FIG. 7 depicts a dipole antenna 40 for use with a packaged SAW sensor for soldering on a top surface of the header. The dipole antenna 40 can be formed on FR4 PCB material 44.
FIG. 8 depicts a SAW sensor 48 soldered to the dipole antenna 40 absent a lid. FIG. 8 also illustrates sensor pins 50 soldered to antenna terminals 54.
FIG. 9 depicts the SAW sensor 48 of FIG. 8 with a lid 58 in place. Note that in both FIGS. 8 and 9 a small horizontal pad 60 at a bottom surface of the sensor 48 provides mechanical stability when the chip carrier 38 is soldered to a pad.
In accordance with common practice, the various described features are not drawn to scale, but are drawn to emphasize specific features relevant to the invention. Like reference characters denote like elements throughout the figures and text.